The present invention relates to the transmission and reception of digital information on multiple lines and at multiple data rates. More particularly, the present invention relates to a device for switching between data communications lines and the like.
As the complexity of data communications networks increases, the need for alternative methods of interfacing the various devices on the networks becomes critical. If one first considers the simplest interconnection between two devices such as two telephones or such as a computer and a terminal, it is evident that a connection between the two devices can be accomplished quite easily. For example, communications can be established between devices with one wire, such as was done with an early telegraph system. Though each device had simultaneous access to the one wire, typically connecting a remote junction box to a central switching location, only one device at a time could use the wire to send messages to other devices. When the device relinquished the line, another device could then use the line to send messages. Since only one device could use the line to send at any one time, the number of messages which could be sent would be quite low. Thus, the data rate of the systems was low. Obviously, some protocol was established for determining which device had the use of the line at any time. Considering the relative simplicity of such a system and the low data rates, and the fact that the system was most likely under the jurisdiction of one entity, such as the telegraph company, the protocol could be as simple as listening to determine whether the line was in use.
When the number of the devices on a system increased and the users became relatively independent, the number of messages to be sent at any one time increased accordingly. A good example of this would be a telephone system. The users in a given area would not all want to be on the same line where each would have to wait until the line to the central switching location was not in use before initiating a message. As was frequently the case, a large number of users could want to send more messages than such a system could handle in a given time frame. Thus, except in areas where this "party line" method was more cost effective and where the number of messages per unit time was low, the "party line" method was replaced with the method of running a communications line from each telephone or other communications device to a central location, thereby replacing the common line to the remote junction box. At the central location, a communications line from one device could be connected to the communications line from another device to which communications was to be established. This could be done manually as with an operator at switchboard, electromechanically such as in a complex telephone crossbar system, or under computer control as is done in modern telephone networks.
Although the method of running communications lines from each user to a central location has many advantages over the party line method, it has the distinct disadvantage of requiring a communications line from each device to a central location. Thus, although two devices might be relatively close to each other compared to the distance to the central location, communications between the two devices would be routed through the central location. In a large, widespread network this would require substantial expenditures for the communications lines. Furthermore, although a device might only be sending and/or receiving messages during a small portion of a given time frame, a dedicated communications line would A/ASR/S be necessary to connect the device to a central location. During the unconnected time, a valuable resource would be idle. If another device has to be added to an existing system a new dedicated communications line would have to be added to connect the device to the central location. It is obvious that this method of interconnecting communications devices has substantial drawbacks when one considers the physical and economical problem of placing the number of communications lines required for a large communications network.
One solution that has developed in the prior art for the problem is the use of time division multiplexing of digital data. Unlike the previously described method, in a system which uses time division multiplexing a communications line is not provided from each device to a central location. Instead, each device is connected to other devices relatively close to it. Thus, there would be a considerable savings in the number of communications lines needed to interconnect the devices in a network. All the devices in a communications network may be connected in a ring, chain or the like, with each device connected to two other devices, or to one other device in the case of a device at the end of a chain connection. One such ring communication system is described in co-pending U.S. patent application Ser. No. 491,551 (now U.S. Pat. No. 4,587,651, issued on May 6, 1986), assigned to the common assignee, the teachings of which are incorporated by reference herein.
Although it might appear that a device would then only be able to communicate with a device to which it has a direct connection, each device can communicate with all other devices connected to the network. The network ring or chain is continuous, with each device either tapping the ring or chain, or forming a part of the ring or chain. It would also appear that the devices are connected much the same as a party line much or the previously described telegraph system. Although the devices are physically connected to the same line at any one time, and do not transmit data at the same time, the devices do not have to wait until other devices complete their messages before sending their own messages. Communication between devices can typically be accommodated using only a portion of the time available in each cyclical message frame, thereby allowing the ring to communicate numerous messages within a given period.
Multiple devices may be connected to a communications line in a ring or chain configuration or the like. In order to coordinate communication between devices in a ring there need be only one or two connections to a controlling unit at some point in the ring or chain. That configuration substantially reduces the need for large amounts of cabling directed to the same location. Moreover, the controlling device no longer has to be able to interconnect the lines from each of the message devices and, therefore, may be less complex. When time division multiplexing is employed the communications line is not assigned solely to one device until the completion of a message. Instead, the line is assigned to each device for a relatively short period of time, typically referred to as a time slot. Other devices in the communications network are likewise assigned to time slots. The time slots occur periodically on the communications line, and are repeated at a frequency such that the device can send or receive data continuously at its normal data rate. A message frame is comprised of all the time slots available for devices. Since each device is assigned to one or more periodic time slots, each device can continuously communicate on the communications line while other devices are similarly communicating via their assigned time slots. Additional devices can be added to the exemplary system by connecting it in the ring or the like without having to run a new communications line to a central location. Thus, many of the physical and economic constraints are no longer a barrier.
In an exemplary system utilizing time division multiplexing, the communications devices might operate at a data rate of 1000 BPS. A communication line operating at 100,000 BPS would be able to transfer messages to or from this device and 99 similar devices in a message frame which has a 1000 hertz repetition rate. The data from each device would be assigned to each of the 100 one-bit time slots in the message frame. Other configurations could assign the time slots for devices in multiple-bit groups.
In either the contemporary centralized network or the contemporary ring network, however, devices can only be added to the system if there are time slots available in a message frame. Therefore, it would be difficult, if not impossible, to add the 101st device to the exemplary system if the available time slots are permanently assigned to other devices. In many communications applications the devices present in the system probably would not all be communicating at the same time. Thus, there could be a substantial number of the time slots idle at any given time. However, a typical prior art communications system would not have the flexibility to reassign the idle time slots to additional devices to take advantage of the available time slots. Although increasing the number of time slots would accommodate extra devices, if possible to do so, the incremental increase in the number of time slots might be large compared to the number of devices to be accommodated, and therefore could result in a large number of unused time slots. Communication of information between data communications devices is typically accomplished with digital data. Essentially, the information is transferred between the devices as a stream of information packets which are represented by voltage levels on the communications line. A "bit" is the minimum amount of information which can be represented by either a high or low voltage level. Information is transferred between communications devices as sequential combinations of bits.
A voice signal can be transferred without any appreciable loss of quality as a stream of 64,000 data bits per second (64,000 BPS). The voice signal is sampled at periodic intervals by the sending device; the samples are converted to a digital format; the digital data is transferred to the receiving device as a stream of data bits; and the digital data is converted to a voice signal by the receiving device.
In comparison to an audio signal, the transmission of character information between a computer and a high-speed video terminal can require data transmission rates in the range of 19,200 BPS. On the other hand, a typical teletypewriter terminal might only require data at a rate of 110 to 300 BPS to operate at full capacity.
Typically a data communications network needs to be capable of handling data rates from 110 BPS to 19,200 BPS, and under same circumstances up to 1,000,000 BPS or more.
Another problem with the typical prior art system utilizing time division multiplexing is that devices operating at different data rates cannot be accommodated. Although the time slot allocations in a message frame might be adequate for most devices in a system, there is often a need for other devices operating at higher or lower data rates. For example, a system might consist primarily of digitized telephones operating at 64,000 BPS. If a typical network is configured to accommodate the telephones, it might not be able to accommodate the communications to and from a terminal device operating at 19,200 BPS. Furthermore, the terminal device might be such that it operates at different rates when communicating with different devices. Thus, a time slot assignment which accommodates a 19,200 BPS data rate would be partially unused if the terminal device were to operate at 9600 BPS or a lower data rate. Similarly, a time slot assigned to a terminal device operating at 9600 BPS would not be able to accommodate the same device operating at 19,200 BPS.
As can be readily seen from the foregoing, the implementation of time division multiplexing in the prior art accomplished a significant savings in physical resources in a typical communications network. However, the prior art systems have serious limitations with regard to flexibility in light of the ever increasing demands on data communications systems, both with regard to the increases in the quantity of devices to be connected to a system and with regard to widespread variations in the communications rates used by those devices.